I started digging into this again, encouraged by some progress I made using Firebug on another front (I found the selector for the submenu,
ul.rMenu-ver).
But I looked around and it looks like the issue is present not just for me but whenever the default menu is used. I looked through the Showing Off section and this site has it too:
www.stevesamps.co.uk > Look how the rollovers work on the Blog menu from Home, versus the Blog menu from Blog.
It's also in the preview of Atahualpa on Wordpress.Org:
http://wordpress.org/themes/atahualpa > Look at the Parent Page menu from Home, versus the Parent Page menu from Parent Page. This is very subtle because there’s only one child page and the colors are very similar. But it’s the same issue.
So, it would be great if this were added to any bug lists that may be flying around. I realize this is minor but it breaks the continuity of the interface.
Meanwhile, in terms of an insert/patch:
It seems to me that what's happening is that when on the parent page, it shows the child links in the menu as "current." I'm using the word "current" from ATO > Menu 1 > "Background color for menu items in hover and current state."
My best guess as to how the “current” state is expressed in the CSS is with
li.current-menu-item. (I tested it by adding an insert to style this yellow with !important but it had no effect.)
But we don’t want to change the style of this element, we want to make sure that the child menu elements aren’t defined as “current” on the parent page. As Juggledad says we need to turn that off.
Any ideas would be much appreciated! I'm learning but I'm way over my head here.